The goal of the proposed research is to examine the initiation and progression of osteoarthritic lesions on the glenohumeral joint surfaces with respect to three distinct phases which describe the natural development of early lesions at the glenohumeral joint. The three phases are: 1) Pre- Osteoarthritic Initiation (Normal Glenohumeral joints) - differences exist in tensile properties of humeral head and glenoid cartilage; 2) Osteoarthritic Initiation Phase -incongruency and excessive humeral head translation produce abnormal contact areas and contact stresses; 3) Osteoarthritic Progression Phase - mismatched articulations damage normal load support mechanism by interstitial fluid pressurization and damage humeral head cartilage; 4) Iatrogenic changes in the glenohumeral joint articulation - surgical tightening of joint capsule produces abnormal contact areas and contact stresses. Each phase will be examined by evaluating visual changes; joint geometry; contact areas and translation during joint motion; tensile, compressive, shear and permeability properties of cartilage at the joint; and three-dimensional finite element analysis of contact stresses correlated with experimental data. The present proposal is an inter-institutional collaborative project between Columbia and Rensselaer and involves the efforts of five senior investigators in biomechanics, finite element analysis, and orthopaedic surgery. In this A2 application, all previously proposed studies regarding patient evaluations, synovial fluid markers, cartilage biochemistry and MRIs have been removed. The study is now narrowly focused on biomechanical testing of 120 glenohumeral joints over four years using experimental methods that have been validated in preliminary studies. The proposal is aimed at understanding a major clinical entity, osteoarthritis of the glenohumeral joint, where there is a dearth of basic scientific effort. It also represents the first attempt to biomechanically test the hypothesis that a correlation exists between regions of high contact stresses and specific sites of cartilage lesions.